We studied the structure of benzene in the last post. However, the structure of benzene has many resonance contributors.
The resonance structures of benzene can be shown as follows –
The first two structures are called Kekule structures and the rest are Dewar structures. The Kekule structures contribute 80% to the resonance hybrid of benzene and the Dewar structures contribute only 20%. Kekule structures are far more stable than Dewar structures. Thus, for all practical purposes, only the Kekule structures are considered as the contributing structures for benzene.
The two Kekule structures are equivalent and thus the stability of the resulting resonance hybrid is very high.
Resonance energy of benzene –
From post 105 we know, that -the difference in energy between the actual molecule and the most stable canonical/resonance structure is called resonance energy.
The resonance energy of benzene is about 36 kcal/mol. This means that the difference in energy between the most stable resonating structure of benzene (it’s Kekule structure – as shown in the above fig) and the actual benzene molecule is 36 kcal/mol. Thus, resonance makes benzene relatively stable in comparison with aliphatic unsaturated compounds.
NOTE – The circle in the benzene molecule represents the π electron cloud.
Thermochemical calculations –
As we know, the benzene molecule has extra stability than expected. This extra thermodynamic stability can be shown by calculating heats of hydrogenation(ΔHh) for three molecules namely –
i)cyclohexene ii) 1,3-cyclohexadiene and iii)benzene.
What is the difference between the above three structures? The number of π bonds increases in each structure. Hydrogenation of these structures means adding hydrogen across the double bonds. Every time hydrogen is added, 28.6 kcal/mol heat(ΔHh) is released in the process (exothermic reaction). Experimentally observation is as follows-
Observed values of heat of hydrogenation.
Less energy means more stability.
Look closely at the values of ΔH. If 28.6 kcal/mol energy is released after hydrogenating one double bond,
i) then for hydrogenating two double bonds in 1,3-cyclobutadiene, the energy released must be 28.6 × 2 = 57.2 kcal/mol. But the observed value is 55.4 kcal/mol as seen in the figure above. Thus, the energy of this molecule is less than expected and thus it is more stable(this stability is due to the conjugation of the double bonds). It is stable by
57.2 – 55.4 = 1.8 kcal/mol.
ii) similarly for three double bonds in benzene, the energy released after hydrogenation, must be 28.6 × 3 = 85.8 kcal/mol. However, the observed value of ΔH is 49.8 kcal/mol. So, benzene has very little energy than expected and thus it is very very stable. It is stable by,
85.8-49.8 = 36 kcal/mol. This is the resonance energy of benzene !!
Also, as benzene has three double bonds in its structure, it was expected to have undergone addition reactions- where substituents add across the double bonds. However, benzene underwent substitution reactions rather than addition reactions. This hinted that the structure of benzene was much more stable than expected. This extra stability was accounted for. How? We shall find that out in our next post.
Till then, be a perpetual student of life and keep learning…
Madoverchemistry 